This invention relates generally to a method for preparation of fused quartz particulates from quartz sand and more particularly to a novel process whereby this product can be produced continuously in a far more effective manner.
Fused quartz particulates or granules are widely used in various diverse product applications including investment casting, refractories, electronic encapsulants, composite fillers, resin fillers and still other products. Typical characteristics for such granular fused quartz material now being manufactured for resin filler, refractory and investment casting applications include a 99.7% SiO.sub.2 content, apparent specific gravity in the range 2.16-2.20, a coefficient of linear thermal expansion in the range 0.5-0.6.times.10.sup.-6 /.degree. C. (20-700.degree. C.), along with varying particle size distributions ranging from 10-325 mesh United States screen size depending upon the particular product specification. The conventional manufacturing process for this type fused quartz material is a non-continuous procedure whereby a batch of the starting mineral sand becomes fused in a rotary electric arc furnace. A description of the conventional furnace and fusion process is contained in U.S. Pat. No. 4,217,462 which can now produce fused quartz ingots as large as 7500 pounds in size accompanied by a considerable ingot "skin" of the unmelted sand which can be a considerable portion of the initial batch charge. Such inherent inefficiency in the current manufacturing process understandably necessitates recycling of the unmelted sand as well as contributes to assorted environmental problems accompanying eventual recovery. Additionally, the cost of large equipment needed to handle and pulverize such large size ingots on a non-continuous basis represents still another serious drawback in the current manufacturing process.
It is also known to manufacture fused quartz rod, ribbon and tubing in a continuous manner from purified quartz sand being fed to a suitable furnace apparatus. Various furnace equipment for this purpose are disclosed in U.S. Pat. Nos. 3,764,286 and 5,026,413 which both employ a refractory metal melting crucible heated electrically by resistance or induction heating means. The molten fused quartz material formed in said crucible member is continuously withdrawn therefrom through a bottom opening while being allowed to slowly cool to ambient temperatures as an elongated solid member (rod, tubing or ribbon). Both prior art patents further mention the employment of a mechanical drawing machine to facilitate continuous removal of the solidifying material. As still further disclosed in both prior art patents, the resulting solid fused quartz product is said to be essentially bubble free with the earlier issued patent pointing out employment of particular furnace reducing gas atmospheres said to enhance bubble removal.
It is one object of the present invention to now provide an improved method whereby fused quartz particulates can be formed in a manner essentially avoiding the above enumerated drawbacks associated with the current manufacturing process.
It is another object of the present invention to provide a novel method enabling continuous formation of said fused quartz particulates at reduced equipment costs.
Still another object of the present invention is to provide a novel method for preparation of fused quartz particulates which employs a thermal fracturing mechanism in producing the desired results.
These and still further objects of the present invention will become more apparent upon considering the following detailed description of the present invention.